I'm not sure if I can help you, but what exactly are you referring to? You mention:
Define "custom mobile application". Is that an Android, iOS, Windows mobile, (god forbid) Blackberry app? So many options to choose from, so little information provided.
There is an iOS SDK available which you can use to create your own custom mobile app for iOS.
The same thing exists for Android.
Doing this with a webapp over HTTP should also be possible, or at least in the very near future:
If you know how to develop for either of those systems, you should be able to use the tools provided to make something work for you. If you haven't got any experience developing for that platform, I'd suggest looking for someone who does, or be prepared to spend some quality time learning it. What you're asking us to do is create an app for you, something which isn't a trivial task, and can require quite some effort and time. To ease up that process, the above mentioned tools have been created. If you don't know how to use those, I'm afraid a step-by-step guide wouldn't really help, since you'd have a hard time understanding what's happening.
That's not to say such a guide won't be made, but I can understand if it's not really a high priority.